Chapter 39 The Atomic Nucleus and Radioactivity Conceptual Physics Hewitt, 1999 Bloom High School

39.1 The Atomic Nucleus Nucleons- particles in the nucleus of an atom Neutrons (n0) & Protons (p+) Almost equal masses Neutrons are “glue” in a nucleus Electrical forces- like repels like p+ repel other p+ in the nucleus Acts over a distance Inverse-square law

Neutrons & Nuclear Strong Forces Acts between nucleons Only acts in close proximity Neutron- nucleon that is unstable when alone More neutrons are needed for more protons

39.2 Radioactive Decay Alpha (a) particle- helium nucleus 2p+ with 2n0 ejected from nucleus Positively charged particle Gamma (g) ray- electromagnetic radiation

Beta emissions b- particle- electron (e-) b+ particle- positron (e+) With extra n0’s, 1n0 is transformed into 1p+ + 1e- Conservation of charges (-) charged particle b+ particle- positron (e+) With extra p+’s, 1p+ is transformed into 1n0 + 1e+ (+) charged particle

39.3 Radiation Penetrating Power g ray- penetrate the most No charge or mass to slow them down Need a very high density substance to block them b particle- penetrates slightly Loses energy with a small number of collisions Thin sheets of metal can block them a particle- penetrates the least Relatively slow and heavy Paper and skin can stop them

39.4 Radioactive Isotopes Atomic number- equal to the number of p+ Carbon, 6p+ Atomic mass number- equal to the number of nucleons Carbon-12 (99% of all carbon) Hydrogen-1 Deuterium-2 (1n0 & 1p+) stable Tritium- 3 (2n0 & 1p+) radioactive

Ions vs. Isotopes Ion- charged particle Gain or loss of an e- through chemical reaction Neutral atom has equal numbers of p+ and e- Isotope- gain or loss of n0 through nuclear reaction Number of p+ must be constant Isotope number (upper number) is (n0 + p+) 235U (U-235) is 92p+ + 143n0

39.5 Radioactive Half-Life Half-Life- amount of time for half of the substance to change or decay If half-life is 1000 years, 50% remains at 1000 years At 2000 years, ½ of ½ remains (25% or ½2) At 3000 years, ½ of ½ of ½ remains (12.5% or ½3)

39.6 Natural Transmutation of Elements Transmutation- changing one element into another Radioactive Decay- can change to another element a-decay- loss of 2p+ Atomic number decreases by 2 Atomic mass decreases by 4 238U  234Th + 4He b--decay- gain of 1p + Atomic number increases by 1 Atomic mass unchanged 234Th  234Pa + 0e- g-decay- no gain or loss of p + No change in atomic number or mass 60Co  60Co + 0g

Calculating a-decay Reactions a-decay- loss of 2p+ Atomic number decreases by 2 Atomic mass decreases by 4 The total mass and atomic number before must equal the total mass and atomic number after the reaction 234Pa  __ + 4He Number of nucleons must be equal 234=x + 4 Number of p+ must be equal 91=Z + 2 Missing element must be 230Ac

Calculating b--decay Reactions b--decay- gain of 1p + Atomic number increases by 1 Atomic mass unchanged The total mass and atomic number before must equal the total mass and atomic number after the reaction 216Po  __ + 0e- Number of nucleons must be equal 216=x + 0 Number of p+ must be equal 84=Z + -1 Missing element must be 216At

Calculating g-decay Reactions g-decay- no gain or loss of p + No change in atomic number or mass The total mass and atomic number before must equal the total mass and atomic number after the reaction 60Co  __ + 0g Number of nucleons must be equal 60=x + 0 Number of p+ must be equal 27=Z + 0 Missing element must be 60Co

39.7 Artificial Transmutation of Elements Elements can be bombarded to change into other elements 14N + 4He  17O + 1H Transuranic element- elements after Uranium Half-life’s are relatively short, so they are not found in nature

39.8 Carbon Dating C-12 very stable (99% of carbon is C-12) C-14 radioactive Found in living tissue and is constantly replaced Ratio of C-12 to C-14 fixed in living tissue In dead tissue, C-14 is not replaced and decreases over time Not found in non-living tissue (metals, rocks, etc.)

39.9 Uranium Dating Uranium- decays in a predictable pattern Lead is a decay product, so is found in all uranium samples

39.10 Radioactive Tracers Radioactive tracer- radioactive isotope used to follow a path Used in agriculture to determine path of fertilizer and water Used in medicine to determine metabolic pathway of medicine or blood

39.11 Radiation and You Radiation naturally occurs everywhere Underground, it’s the cause of warmth Radiation more strong at high altitudes because there is less atmospheric shielding from cosmic radiation

Lab 97: Nuclear Marbles Groups of 4 Computed diameter Procedure Step 1 3 metersticks Computed diameter P=2N(R+r)/L P=H/T Replace (R+r) with “d,” combine formulae and solve for d Procedure Step 1 200 trials DON’T AIM for the marbles, aim for 1cm, then 2cm, etc. and go back and forth until you have 200 trials

Lab 98: Half-Life Groups of 4 Procedure Step 1 Data Table A 50 pennies Procedure Step 1 Keep the “heads” pennies out of the box! Data Table A Number of Pennies removed it the TOTAL removed, not just from that trial Analysis 4: Percent error (Analysis#2-theory)/theory x 100 = %